Make RenameDialog modal again.

Summary:
The rename dialog used to be modal, but commit
828ba8902ce16819a385832b487e60eab36e54ca changed the behavior without
really explaining why.

Test Plan: disable inline renaming and rename something.

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D17621